1. Field of the Invention
Embodiments of the present invention relate generally to processes for producing heavy oil. Various embodiments of the present invention are particularly useful in producing heavy oil emulsions that can be used in boilers in steam assisted gravity drainage (SAGD) processes for recovering heavy oil.
2. Description of the Related Art
Heavy oil is naturally formed oil with very high viscosity but often contains impurities such as sulfur. While conventional light oil has viscosities ranging from about 0.5 centipoise (cP) to about 100 cP, heavy oil has a viscosity that ranges from 100 cP to over 1,000,000 cP. Heavy oil reserves are estimated to equal about fifteen percent of the total remaining oil resources in the world. In the United States alone, heavy oil resources are estimated at about 30.5 billion barrels and heavy oil production accounts for a substantial portion of domestic oil production. For example, in California alone, heavy oil production accounts for over sixty percent of the states total oil production. With reserves of conventional light oil becoming more difficult to find, improved methods of heavy oil extractions have become more important. Unfortunately, heavy oil is typically expensive to extract and recovery is much slower and less complete than for lighter oil reserves. Therefore, there is a compelling need to develop a more efficient and effective means for extracting heavy oil.
Heavy oil that is too deep to be mined from the surface may be heated with hot fluids or steam to reduce the viscosity sufficiently for recovery by production wells. One thermal method, known as steam assisted gravity drainage (SAGD), provides for steam injection and oil production to be carried out through separate wells. The optimal configuration is an injector well which is substantially parallel to and situated above a producer well, which lies horizontally near the bottom of the formation. Thermal communication between the two wells is established by preheating the area between and around the injector well and producer well. Generally, such preheating is by steam circulation until the reservoir temperature between the injector and producer wellbore is at a temperature sufficient to drop the viscosity of the heavy oil so that it has sufficient mobility to flow to and be extracted through the producer well. Typically, preheating involves introducing steam through both the injector well and producer well. Steam circulation through the injector well and producer well will occur over a period of time. At some point before the circulation period ends, the temperature midway between the injector and producer will reach about 80 to 100° C. and the heavy oil will become movable (3000 cP or less). Once this occurs, the steam circulation rate for the producer well will be gradually reduced while the steam rate for the injector well will be maintained or increased. This imposes a pressure gradient from high, for the area around the injector well, to low, for the area around the producer well. With the oil viscosity low enough to move and the imposed pressure differential between the injection and production wellbores, steam (usually condensed to hot water) starts to flow from the injector into the producer. As the steam rate is continued to be adjusted downward in the producer well and upward in the injector well, the system arrives at steam assisted gravity drainage operation with no steam injection through the producer well and all the steam injection through the injector well. Once hydraulic communication is established between the pair of injector and producer wells, steam injection in the upper well and liquid production from the lower well can proceed. Due to gravity effects, the steam vapor tends to rise and develop a steam chamber at the top of the region being heated. The process is operated so that the liquid/vapor interface is maintained between the injector and producer wells to form a steam trap which prevents live steam from being produced through the producer well.
Once the formation has been preheated, SAGD operation can commence. In operation of the SAGD process, steam will come into contact with the heavy oil in the formation and, thus, heat the heavy oil and increase its mobility by lessening its viscosity. Heated heavy oil will tend to flow downward by gravity and collect around the producer well. Heated heavy oil is produced through the producer well as it collects. Steam contacting the heavy oil will lose heat and tend to condense into water. The water will also tend to flow downward toward the producer well and is produced with the heavy oil. Such produced water may be treated to reduce impurities and reheated in the boiler for subsequent injection.
Steam-based heavy oil recovery processes, such as SAGD processes described above, are most likely to burn natural gas as the fuel of choice to produce high-pressure steam for bitumen recovery. Steam requirements for such processes are on the order of two to five times as much steam as recovered oil. Thus, the cost of producing steam is one of the greatest operating expenses of recovery; the overall cost is greatly affected by the price of fuel used in producing steam. Thus, the use of natural gas as a fuel for producing steam reduces operating cost when the price of natural gas is low but these costs will increase proportionally as the price of natural gas increases. As a result, interest in alternative fuels is particularly kindled when the price of natural gas increases.